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Abstract

We demonstrate that the pump’s spatial input profile can provide additional degrees of freedom in tailoring at will the nonlinear dynamics and the ensuing spectral content of supercontinuum generation in highly multimoded optical fibers. Experiments and simulations carried out at 1550 nm indicate that the modal composition of the input beam can substantially alter the soliton fission process as well as the resulting Raman and dispersive wave generation that eventually lead to supercontinuum in such a multimode environment. Given the multitude of conceivable initial conditions, our results suggest that it is possible to pre-engineer the supercontinuum spectral content in a versatile manner.

Figures (7)

Fig. 1 (a) Energy distribution among the LPlm modes of a parabolic fiber when excited on-axis. In this case, only the LP0m modes are populated with the fundamental mode taking most of the energy. (b) Modal population when the system is excited off-axis. For this input, considerable energy resides in the LP1m set.

Fig. 2 (a) Evolution of the supercontinuum spectrum in a 20 cm long parabolic MMF when the pulse energy is 150 nJ for (a) an on-axis excitation (b) off-axis input (offset by 10 µm). In all cases three stages are apparent: (i) initial spectral broadening, (ii) soliton fission, and (iii) soliton and dispersive wave propagation. The respective distances where soliton fission occurs are also shown. In (b) the circles denote the onset of the first three GPI sidebands. The pump wavelength is 1550 nm.

Fig. 3 Comparison of the supercontinuum spectra features produced in an anomalously dispersive parabolic MMF for on-axis (blue) versus off-axis (red) excitation conditions. The propagation distance is 20 cm and the pulse width is 500 fs. All other parameters are the same as Fig. 2.

Fig. 4 (a) Resulting temporal features after 20 cm in a parabolic MMF when excited on axis. (b) Temporal evolution of the initial 500 fs pulse (150 nJ) corresponding to (a). The emergence of slow solitons is apparent. (c) and (d) Same as in (a) and (b), respectively, for off-axis excitation. (e) The spatial intensity profile and its x-cross section corresponding to the slowest dominant soliton at the end of the fiber, when illuminated with an on-axis Gaussian beam. (f) Same as in (e) for off-axis launching conditions, where the soliton-beam experiences transverse oscillations during propagation.

Fig. 5 Experimentally measured (a) NIR and (b) visible supercontinuum spectrum for on-axis excitation together with generated transverse output intensity profiles after 1 m of propagation. (c) and (d), Same as in (a) and (b) for an off-axis excitation. In all cases, each division represents a 10 dB variation. The scale bars in the insets represent 20 µm.

Fig. 6 Generated (a) NIR and (b) visible portion of the spectrum when the fiber is excited by a ring beam. Same as in (c) and (d) when using a two-spot excitation. Each division in (a) and (c) represents a 20 dB differential while in (b) and (d) a 10 dB variation.

Fig. 7 Experimentally measured NIR and visible supercontinuum spectra for four different initial spatial conditions together with generated transverse output intensity profiles after 1m of propagation. In all cases the pulse energy is 150 nJ. In all cases, each division represents a 10 dB variation. The scale bars in the insets represent 20 µm.